1. Field of the Invention
The present invention relates to an image forming method useful for electrophotographic image forming apparatuses such as copiers, printers and facsimile machines, and more particularly to an image fixing method of full color toner images.
2. Discussion of the Background
Various electrophotographic full color image forming methods using three color toners of yellow, magenta and cyan toners, or four color toners of yellow, magenta, cyan and black toners have been proposed and practically used.
When the surface of color toner images is smooth, the toner images have high gloss because the surface of the toner images reflects light like a mirror. On the contrary, when toner images have rough surface, the toner images look frosted because light randomly reflects at the surface of the toner images. Therefore, when full color toner images are fixed by application of heat, the color toner images are typically fixed at a relatively high temperature to decrease the melt viscosity of the color toners constituting the color images, i.e., to prepare toner images having high gloss.
However, the gloss of the full color images prepared by the above-mentioned method is not satisfactory because the thus prepared black color image has too high a gloss, since the black toner tends to absorb heat more than the other color toners. Therefore, the color images are hard to see, and have low visual qualities.
In addition, when color images are fixed with a fixing roller at a temperature at which the color toners constituting the color images have low melt viscosity, an offset problem in which the color toners adhere to the fixing roller tends to occur.
A release agent, such as oils, is typically applied to the fixing roller to avoid the offset problem. Therefore, a release agent applying device is needed, and other problems occur, such that the image forming apparatus becomes large in size and manufacturing costs of the image forming apparatus increase.
Further, when the fixing method in which the fixing temperature is increased is used, the power consumption of the image forming apparatus increases. Therefore a requirement of low power consumption (i.e., a requirement to save energy) cannot be satisfied.
Because of these reasons, a need exists for an image forming method by which color toner images having uniform high gloss are stably obtained without causing the offset problem and without increasing manufacturing cost of the image forming apparatus.
Accordingly, an object of the present invention is to provide an image forming method by which color toner images having uniform high gloss are stably obtained without causing the offset problem and without increasing manufacturing cost of the image forming apparatus.
Briefly this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by an image forming method including the steps of forming a color image which is formed on a support and which comprise a plurality of color toners each having a melting point or a softening point; heating the color image on the support with a fixing member upon application of pressure while the color image contacts a surface of the fixing member such that the color image is heated at a temperature not lower than the melting points or softening points of the plurality of color toners to melt or soften the color toners; and then cooling the color image to a temperature lower than the melting points or softening points while the color image contacts the surface of the fixing member; and then separating the color image from the fixing member, wherein the surface of the fixing member has a ten-point mean roughness not greater than 20 xcexcm.
In the heating step, the pressure P (9.8xc3x97100 N/m) applied to the color image preferably satisfies the following relationship:
((9/5)xc3x97logxcex7xe2x88x924/5)/15xe2x89xa7Pxe2x89xa7((1/100)xc3x97logxcex7)/15 
wherein xcex7 represents the melt viscosity of each of the color toners in units of centipoise at the temperature in the heating step.
The melt viscosities of the plurality of color toners are preferably from 10 to 1013 centi-poise.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.